Methylxanthines and benzodiazepines are among the most widely used behaviorally active drugs in contemporary society. Research is proposed to systematically investigate pharmacological and environmental factors that mediate their effects on behavior. The behavioral actions of methylxanthines will be investigated by: 1) determining their effects on schedule-controlled behavior of monkeys under conditions in which caffeine produces marked increases in response rate, 2) directly comparing their effects with those of prototypical psychomotor-stimulant drugs, 3) determining their effects in combination with selected derivatives of adenosine that block methylxanthine-induced increases in responding, and 4) evaluating their reinforcing effects under conditions in which caffeine maintains persistent self-administration. The behavioral actions of benzodiazepines and of newly developed drugs with putative antianxiety activity will be investigated by: 1) determining their effects on schedule-controlled behavior under conditions that distinguish antiznxiety drugs from other pharmacologic classes, 2) evaluating their stimulus effects under conditions in which benzodiazepines exert discriminative control over behavior, and 3) determining their effects in combination with selective benzodiazepine antagonists. The interactions between antianxiety drugs, methylxanthines, and derivatives of adenosine will be investigated under conditions in which methylxanthines enhance and adenosine derivatives block the characteristic effects of benzodiazepines. The proposed studies will provide needed information about how methylxanthines and benzodiazepines alter and control behavior.